Over the past several years, significant progress has been made in our understanding of the etiology and pathogenesis of periodontal diseases. However, the nature and contribution of the immune system to these disorders remain unclear. It is our hypothesis that the immune system plays a primary role to minimize and/or prevent infection. Furthermore, we propose that immunoregulatory abnormalities contribute to the pathogenesis of and susceptibility to periodontal disease. In this regard, our investigations have demonstrated that several periodontal pathogens produce factors capable of suppressing human T- and B-cell function; these include Actinobacillus actinomycetemcomitans, Fusobacterium nucleatum, Treponema denticola and Prevotella intermedia. The fundamental hypothesis of our studies is that periodontal pathogens produce immunosuppressive factors (ISFs) that mediate local and/or systemic immunosuppression, thereby enhancing their own virulence and/or that of other opportunistic microorganisms. To conduct this study, we plan to utilize two general strategies. In the first of these approaches, we will focus on the target cells treated with ISF. Methods will be employed to detail the mechanisms by which the ISFs affect these cells. In the second approach, we will focus on the ISFs themselves. Experiments described will further characterize the genes encoding for the ISFs and examine the activities of the recombinant peptides. The study is composed of four Specific Aims: (1) To characterize and define the mechanism by which the A. actinomycetemcomitans ISF induces a unique subpopulation of human T-cells: CD4+CD8+ dual positive cells; (2) To determine the functional properties of these dual positive T-cells. Specifically, we will extend our preliminary findings and determine if these suppressor cells function by virtue of their ability to kill target cells; (3) To define the mechanism by which F. nucleatum ISF and T. denticola ISF arrest cells in the G1 phase of the cell cycle; and (4) To characterize the genes encoding the A. actinomycetemcomitans and F. nucleatum ISFs and to examine the function and activities of the individual F. nucleatum ISF subunit peptides. Expression systems which allow for the rapid and simple purification of ISF from E. coli will also be developed. In the long term, it is anticipated that our studies will contribute important knowledge that extends our understanding of the etiology and pathogenesis of periodontal disease. Specifically, these studies will define the host-parasite interactions that occur in the development and/or resolution of these disorders.